1. Field of the Invention
The present invention relates generally to an illumination unit and a liquid crystal display device including the illumination unit, and more particularly to a structure for aligning an optical sheet of an illumination unit with a light guide plate and keeping the alignment state.
2. Description of the Related Art
A technique for fixing an optical sheet to a light guide plate has been proposed (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2006-201210). According to this patent document, bosses (projections), which are integrally formed on a back surface of a light guide plate, are passed through through-holes of a reflection sheet, and inserted in boss-receiving holes provided in a metallic back-side casing. Thus, the reflection sheet is directly fixed to the back surface of the light guide plate.
In recent years, in order to increase the luminance of the liquid crystal display device, there has been a need to efficiently guide light, which is emitted from a light source, to a light guide plate. Thus, there is a tendency that the thickness of the light guide plate increases. It is very difficult to fabricate a thick light guide plate by injection molding. Under the circumstances, in particular, a light guide plate, which is applied to a liquid crystal display device with a relatively large screen size, is fabricated, in most cases, by a cutting process by cutting-out from a plate material.
However, it is difficult to fabricate the light guide plate having bosses on the back surface, as disclosed in the above-described patent document, by a cutting process. It is thus difficult to align the light guide plate, which is fabricated by the cutting process, with the optical sheet. Besides, for example, a separate member, such as a double coated adhesive tape, is needed in order to fix the optical sheet to the light guide plate in the state in which the optical sheet is aligned with the light guide plate, leading to such problems as complexity in structure, complexity in a fabrication step for fixation, and an increase in manufacturing cost.